1. Field of the Invention
The present invention relates generally to a color video projecting apparatus. More particularly, the present invention relates to a color video projecting apparatus for projecting a color image on a projection screen in which monochrome images of different colors are produced on a plurality of cathode ray tubes and projected and composed as a color image on the projection screen through optical systems such as projecting lenses with minimum misalignment.
2. Description of the Prior Art
It is well known in the art to utilize color video projecting apparatus as shown in FIGS. 1 to 3 in which the present invention can be utilized. In this apparatus, a red color cathode ray tube 1R and a green color cathode ray tube 1G are located in opposing relation to a projection screen 2, and a blue color cathode ray tube 1B, facing up, is located under the cathode ray tubes 1R and 1G, respectively, as shown in FIGS. 2 and 3. The light from an image on the red color cathode ray tube 1R passes through a half mirror 3, and then through a lens 4R by which it is projected on the projection screen 2. The light from an image on the green color cathode ray tube 1G passes through a lens 4G and is then projected on the screen 2. The light from an image on the blue color cathode ray tube 1B is reflected on the half mirror 3, passes through the lens 4R, and is then projected on the screen 2.
In this apparatus, optical or light axes 5R and 5G of the lenses 4R and 4G are directed to a center O.sub.s of the projection screen 2. Cathode ray tubes 1R, 1G and 1B are arranged so that the angles between a normal 6 to the screen 2 and the light axes 5R and 5G in the horizontal direction are both equal to .omega., i.e., the light axes 5R and 5G are symmetric with respect to the normal 6, as shown in FIG. 1. In addition, the angle between the normal 6 and the light axes 5R and 5G in the vertical direction is selected as .alpha.. Thus, in utilizing the prior art apparatus, the red and blue color video image lights are composed by the half mirror 3, this compounded light being composed with the green color video image light on the screen 2 to form a color image.
In the above prior art apparatus, the light axis 5R of the lens 4R and the light axis 5G of the lens 4G are inclined at the angle .omega. in the horizontal direction and the angle .alpha. in the vertical direction with respect to the normal 6 of the projection screen 2, as described above. In this manner, the light axes 5R and 5G are not perpendicular to the screen 2, and the resulting image composed on the screen 2 is distorted in a keystone configuration which is asymmetric with respect to the horizontal center line appearing in the image on the screen 2.
To avoid this distortion, the prior art devices pre-distort the rasters on the cathode ray tubes 1R, 1G and 1B in the reverse direction to the key stone configuration, as shown in FIGS. 4A and 4B, to compensate for the key stone distortion on the screen 2. FIGS. 4A and 4B show a schematic representation of the superposition of the raster on the cathode ray tubes 1G and 1R such that a vertical line 10 of the raster intersects a center 0 of the phosphor plate of each of the cathode ray tubes, and the images on the respective cathode ray tubes 1G and 1R are symmetric with respect to an imaginary vertical center line 11 on each of the cathode ray tubes. The image on the cathode ray tube 1B is the same as the image on the cathode ray tube 1R. In this manner, the cathode ray tubes 1R, 1G and 1B are arranged so that the images on the cathode ray tubes 1R, 1G and 1B are projected on the screen 2 with the center 0 of each of the cathode ray tubes being projected on the center O.sub.s of the screen 2.
Each of the cathode ray tubes 1R, 1G and 1B, in the prior art apparatus, has a flat front panel or phosphor screen, as shown in FIG. 5. In this manner, even if the deflection angles .theta..sub.1 and .theta..sub.2 of the cathode ray tubes are equal at the center and edge portions of the front panel or phosphor screen, the respective lengths L.sub.1 and L.sub.2 of the scanning lines on the phosphor screen are not equal. This difference in the lengths of the scanning lines becomes large at the near edge of the phosphor screen, as shown in the graph of FIG. 6.
In the instance where the images are pre-distorted, as shown in FIGS. 4A and 4B, left and right lengths a.sub.1 and a.sub.2 of the scanning line, which are measured from the vertical line 10 in the original image, are equal at their upper edges. In addition, left and right lengths b.sub.1 and b.sub.2 of the scanning line, which are measured from the center line 11, are not equal at their upper edges. As a result, the image on the cathode ray tube 1G is enlarged at its right upper end, while the image on the cathode ray tube 1R is enlarged at its left upper end. Therefore, when these images are composed on the screen, they are not coincident with each other at the upper left and right corners with the result that misalignment occurs and a color picture of high quality can not be obtained.